Exploration of physical principles underlying lipid regular distribution: effects of pressure, temperature, and radius of curvature on E/M dips in pyrene-labeled PC/DMPC binary mixtures

Abstract

In a previous study, we observed a series of dips in the plot of E/M (the ratio of excimer to monomer fluorescence intensity) versus the mole fraction of 1-palmitoyl-2-(10-pyrenyl)decanoyl-sn-glycerol-3-phosphatidylcholine (Pyr-PC) in Pyr-PC/DMPC binary mixtures at 30 degrees C. In the present study, we have characterized the physical nature of E/M dips in Pyr-PC/DMPC binary mixtures by varying pressure, temperature, and vesicle diameter. The E/M dips at 66.7 and at 71.4 mol% PyrPC in DMPC multilamellar vesicles remain discernible at 30-43 degrees C. At higher temperatures (e.g., 53 degrees C), the depth of the dip abruptly becomes smaller. This result agrees with the idea that E/M dips appear as a result of regular distribution of pyrene-labeled acyl chains into hexagonal super-lattices at critical mole fractions. Regular distribution is a self-ordering phenomenon. Usually, in self-ordered systems, the number of structural defects increases with increasing temperature, and thermal fluctuations eventually result in an order-to-disorder transition. The effect of vesicle diameter on the E/M dip at 66.7 mol% Pyr-PC in DMPC has been studied at 37.5 degrees C by using unilamellar vesicles of varying sizes. The E/M dip is observable in large unilamellar vesicles; however, the depth of the E/M dip decreases when the vesicle diameter is reduced. When the vesicle diameter is reduced to about 64 nm, the dip becomes shallow and split. This result suggests that the curvature-induced increase in the separation of lipids in the outer monolayer decreases the tendency of regular distribution for pyrene-labeled acyl chains. Regular distribution is believed to arise from the long-range repulsive interaction between Pyr-PC molecules due to the elastic deformation of the lipid matrix around the bulky pyrene moiety. When the radius of curvature becomes small, outer monolayer lipids are more separated. Therefore, pyrene-containing acyl chains fit better into the membrane matrix, which alleviates the deformation of the lattice and diminishes the long-range repulsive interactions between pyrene-containing acyl chains. Furthermore, we have shown a striking difference in the pressure dependence of E/M at critical Pyr-PC mole fractions and at noncritical mole fractions. In the pressure range between 0.001 and 0.7 kbar at 30 degrees C, E/M decreases steadily with increasing pressure at noncritical mole fractions; in contrast, E/M changes little with pressure at critical mole fractions (e.g., 33.3 and 50.0 mol% Pyr-PC). The pressure data suggest that membrane free volume in the liquid crystalline state of the bilayer is less abundant at critical Pyr-PC mole fractions than at noncritical mole fractions.

Binding of Adriamycin to Liposomes as a Probe for Membrane Lateral Organization

“…Our laboratory provided the first evidence for the formation of superlattices in fluid binary alloys (Somerharju et al, 1985). These findings were subsequently confirmed and extended to other compositions (e.g., Tang and Chong, 1992;Chong et al, 1994;Tang et al, 1995). Notably, these superlattices were mainly concluded from steady-state fluorescence measurements, and the underlying repulsive potential was suggested to be steric elastic strain imposed by the bulky fluorescent moities.…”

Effect of Hydration and Packing Order on Large Unilamellar Vesicle Fusion: The Role of Cholesterol

“…Several conceptual theories explain these phenomena as the superlattice model or the umbrella model 25 . In the superlattice theory, cholesterol molecules tend to be regularly distributed in a hexagonal host lipid lattice; however, irregular distributions always coexist with regular distributions and result in very small segregated domains [26][27][28][29] . According to this model, the ratio of regular to irregular regions reaches local maxima at critical cholesterol molar fractions, and we suggest that in these states, the domain boundary surfaces also reach local maxima 30,31 .…”

Evidence for phospholipid microdomain formation in liquid crystalline liposomes reconstituted with Escherichia coli lactose permease

“…Formation of lipid domains in liquid crystalline bilayers, resulting from the mismatch of the effective lengths of the hydrophobic parts of the constituent phospholipids, has recently been described (Lehtonen et al, 1996). To this end, there is experimental evidence revealing organization of fluid binary lipid mixtures into regular superlattice arrays (Berclaz and McConnell, 1981;Somerharju et al, 1985; Kinnunen et al, 1987;Tang and Chong, 1992;Chong et al, 1994;Tang et al, 1995).…”